In the following description specific reference will be made to the application of single-phase power dc-ac converters for energy photovoltaic sources known as so-called “grid-connected” systems (connected to an alternate current electric energy distribution network), but it should be understood that the inverter device (and the related controlling method) according to the invention may be applied to both single-phase and multi-phase converters for any energy source, in particular renewable, such as fuel cells, wind turbines, and other sources having a variable maximum power point, or more generally a maximum energy convenience point maximum energy convenience point, at the input, and a constrained PF-out at the output. More in general, the inverter device according to the invention may be applied to converters for any energy source that is characterised by the existence of particular specific operation conditions which are deemed preferential, in relation to produced energy, energy efficiency, component stress degree, life, or any other evaluation factor may be defined for a specific source, and which conditions are variable, due to climatic or physical factors or factors of any nature, which are either controllable or uncontrollable, either predictable or unpredictable, and identifiable through a particular point of one of the source output electrical characteristic curves such as power-voltage, power-current, voltage-current, current-voltage, efficiency-voltage, efficiency-current or other ones similar thereto.
It is known that electronic power converters used in photovoltaic systems called “grid-connected” achieve the dual function of extracting the maximum power from the photovoltaic field (MPPT) and delivering the extracted power to the alternate current or AC network with high output Power Factor (PF-out), wherein the current is in phase with the voltage and presents a low harmonic distortion.
The market presently offers several photovoltaic inverters, generally based on a double stage architecture, of the type shown in FIG. 1 for a single-phase application, interposed between the photovoltaic source 1 and the ac electric energy distribution grid 2. Such architecture presents as main advantage some ease of design and implementation thanks to the separation of the two functionalities of MPPT and dc-ac conversion with PF-out control: the first one is achieved by the dc-dc converter 3, controlled by the maximum power point tracking MPPT module 4 connected to the output of the source 1, while the functionality of dc-ac conversion is achieved through the dc-ac converter 5, controlled by the module 6 connected to the input of the grid 2.
Through a careful design of the controller of each stage, it is possible to attain satisfactory performance in terms of both efficiency of energy extraction from the photovoltaic field 1 and electric efficiency of conversion from the photovoltaic field 1 to the grid 2.
However, such system presents some drawbacks and limitations, such as: the high costs of the components of the two power stages; the performance decay at low levels of current; the basically slow dynamics due to the passage of energy through the dc bus sustained by a high-capacity bulk capacitor 7; the need of stabilising the bulk voltage without suppressing the grid second harmonic ripple.
Since the cost of a photovoltaic system is made up by about 50% by the cost of the photovoltaic modules and by 30-40% by the photovoltaic inverter, reducing the cost of the latter may be surely an interesting incentive to a wider diffusion of the use of renewable sources for producing electric energy, thus meeting a diffused social, economic, and environmental need.
Consequently, in the last years some devices have been developed attempting to solve the problem of the costs of a photovoltaic inverter. Such solutions are based on:                the use of single stage inverters, wherein the functionalities of MPPT, dc-ac inversion, and PF-out control are integrated in a single power circuit;        the development of integrated control techniques allowing to achieve the combined function of MPPT, dc-ac inversion, and PF-out control; and        the implementation of control circuits integrated on microchip, developed and optimised for ensuring the achievement of the functionalities necessary to the single stage photovoltaic inverter in a robust and adaptive way with respect to the characteristics of the source photovoltaic field.        
In particular, Y. Chen and K. Ma. Smedley, in “A cost-effective single-stage inverter with maximum power point tracking”, IEEE Transactions on Power Electronics, Vol. 19, No. 5, September 2004, pp. 1289-1294, have recently proposed the application of the OCC control, as shown in FIG. 2, to a single stage inverter for photovoltaic use, wherein a dc-ac converter, connected at its input to the photovoltaic source 1 and at its output to the grid 2, comprises four semiconductor power switches M1-M4 (preferably implemented through respective MOSFETs or IGBTs). The single stage inverter is controlled by a driving unit 8, that operates on the basis of values of certain electric quantities at input and output of the same converter, through the One-Cycle Control technique, for controlling the voltage of a converter over a single switching cycle. More in detail, as also disclosed by K. M. Smedley, S. Cuk, “One-cycle control of switching converters”, Power Electronics, IEEE Transactions on Volume 10, Issue 6, November 1995 pp. 625-633, the OCC technique is a non linear control technique that offers significant advantages in terms of line noise rejection and response speed, that is based on the integration function of a suitable variable (voltage or current), with switching waveform, in order to impose its average value equal to a value indicated by a control reference signal (in particular a control voltage Vc).
However, the scaling of the OCC controller 19 of FIG. 2, intended for a photovoltaic MPPT application, requires an adequate design/circuit approach and an accurate setting of the circuit parameters, in order to be able to really achieve both the MPPT control and the PF-out optimisation with performance comparable to that of a present good performance double stage system. In the system of FIG. 2, power passes from the photovoltaic field 1 to the grid 2 through the inverter, that operates as a so called buck converter (i.e. a dc-dc converter that provides for an output average voltage less than the input dc voltage by varying the duty cycle of a switch connecting the input to the output, i.e. the ratio between the time during which the switch is closed and the period of the periodic signal controlling the switch) in each half cycle of the frequency of the line 2 thanks to the control circuit 8.
By observing FIG. 2, the inner pulse width modulation PWM loop is characterised by a high speed and it determines, cycle by cycle, the duty-cycle value necessary for obtaining a quasi-sinusoidal output current following the waveform of the line ac voltage vo(t). The outer loop, instead, is intended for the MPPT function and it adjusts the output power according to the maximum power that may be extracted from the photovoltaic field 1.
In the following of the present description and in the claims, the following correspondences between symbols and electric quantities will be used:                vg(t) indicates the instant voltage generated by the photovoltaic field 1;        vm(t) indicates a voltage instant value defined by the following equation        
            v      m        ⁡          (      t      )        =            (                                    V            c                    -                                    K              g                        ·                                          v                g                            ⁡                              (                t                )                                                                          R            1                    ⁢                      C            1                              )        ·          T      s                      where Kg is a constant, Ts is the switching period of the power stage, i.e. of the inverter switches M1-M4, and the other quantities are immediately comprehensible on the basis of FIG. 2;        vo(t) indicates the instant value of the voltage of the grid 2;        io(t) indicates the instant value of the inverter output current;        d(t) indicates the instant value of the inverter duty cycle;        vg, vm, vo, io, and d indicate the instant average values, i.e. the average values calculated over a switching period Ts, of the respective instant variables vg(t), vm(t), vo(t), io(t), and d(t);        Vg and Vm indicate the average values of the corresponding instant variables vg(t) and vm(t), calculated over a period Tgrid of the voltage vo(t) of the grid 2; and        Vo and Io indicate the effective values of the corresponding instant variables vo(t) and io(t), calculated over a period Tgrid of the voltage vo(t) of the grid 2.        
Still making reference to FIG. 2, as disclosed by Chen and Smedley, the OCC control ensures a high PF-out if the output current io(t) is proportional to the grid voltage vo(t), that is if:io=(K1−K2)·vo  [1]where K1 and K2 are positive constants, the values of which determine the operation power levels of the inverter.
By multiplying equation [1] by the sensing resistance Rs and taking account of the conversion ratio of the buck converter, equal tod=vo/Vg where d is the duty cycle and Vg is the average value of the dc voltage of the photovoltaic field 1 over a period Tgrid of the voltage vo(t) of the grid 2, equation [1] becomes:Rs·io=Rs·K1·vo−Rs·K2·vo=RsK1·vo−Rs·K2·vg·d=K·vo−vm·d  [2]whereK=Rs·K1 andvm=Rs·K2vg  [3]Consequently:K·vo−Rs·io(t)=vm·d  [4]
Equation [4] constitutes the basic relation for achieving the OCC control through the inner bop of FIG. 2.
The average output power Po may be derived from equations [2][3] and [4]:
                              P          o                =                                            V              o                        ·                          I              o                                =                                                    V                o                            ·                              (                                                      K                    -                                                                  V                        m                                                                    V                        g                                                                                                  R                    s                                                  )                            ·                              V                o                                      =                                                            V                  o                  2                                                  R                  s                                            ·                              (                                  K                  -                                                            V                      m                                                              V                      g                                                                      )                                                                        [        5        ]            where Vo and Io are the effective values of the output voltage and current, vo(t) and io(t), respectively. From FIG. 2 and equation [3] it follows:
                              v          m                =                              (                                                            V                  c                                -                                                      K                    g                                    ·                                      v                    g                                                                                                R                  1                                ⁢                                  C                  1                                                      )                    ·                      T            s                                              [        6        ]            where Kg is a constant, R1·C1 is the time constant τ of the integrator circuit 9, and Vc is the control voltage.
Hence, the output power Po is equal to:
                              P          o                =                                            V              o              2                                      R              s                                ⁢                      (                          K              +                                                                    K                    g                                    ⁢                                      T                    s                                                                                        R                    1                                    ⁢                                      C                    1                                                              -                                                                    V                    c                                    ⁢                                      T                    s                                                                                        V                    g                                    ⁢                                      R                    1                                    ⁢                                      C                    1                                                                        )                                              [        7        ]            
Equation [7] gives the inverter output power Po as a function of parameters K, Kg, Vc, R1, C1, Rs and Ts. In particular, it indicates that it is necessary to adequately choose the aforesaid parameters in order to maximise the inverter output power Po, i.e. the output power of the photovoltaic field 1.
For a stable operation of the OCC controller 19, Chen and Smedley have indicated the following conditions:
                                          R            1                    ⁢                      C            1                          <                  T          s                                    [        13        ]                                          V          c                ≥                                            K              g                        ⁢                          V              g                                +                                                    (                                                      2                    ⁢                                                                                  ⁢                                          V                                              o                        ,                        max                                                                              -                                      V                    g                                                  )                            ⁢                              R                s                            ⁢                              R                1                            ⁢                              C                1                                                    2              ⁢                                                          ⁢              L                                                          [        14        ]            where Vo is the control voltage and Vo,max is the maximum value assumed by the output voltage vo(t). Therefore, the parameters characterising the OCC controller 19 of the system of FIG. 2, determining its performance in photovoltaic applications, are the operative parameters Kg, Vc, and K, and the circuit parameters R1, C1, Rs.
However, the converter proposed by Chen and Smedley, illustrated with reference to FIG. 2, suffers from some drawbacks, due to the fact that the above constraints [13] and [14] do not ensure the real operation of the photovoltaic single stage inverter with maximum power point tracking (MPPT) and high PF-out in any operation condition, such as for instance in variable sunlight conditions.
In fact, in order to obtain a robust PF-out control it is necessary that, at any sun irradiance level S within the operation range [Smin, Smax] that is intended to ensure, the OCC controller 19 is capable to properly modulate the output current io(t) according to equation [4] and the output power Po according to equation [5]. The average power Po is modulated by the term (K−Vm/Vg) depending on the irradiance level S through the term Vm/Vg. Such ratio appears in equation [5] and hence the oscillations of the voltage of the photovoltaic field 1 translate into a perturbation of the term vm(t)/vg(t) that strongly affects the waveform of the output current io(t).
In other words, Chen and Smedley have proposed design equations allowing infinite solutions (though corresponding to levels of efficiency of extraction of power from the photovoltaic field lower than the maximum one), but they have not been capable to give explicit and defined guidelines for setting the aforementioned parameters, nor systematic treatment of the problem and solution strategies are available in the technical-scientific literature. In fact, Chen and Smedley have presented only one example of solution based on the trial-and-error approach, that is of poor both technical-scientific and application interest, since it is the result of a non systematic approach and it does not give results ensuring good performance.
Therefore, the OCC technique applied to the nowadays available photovoltaic single stage inverter does not include in a reliable and efficient way the MPPT functionality simultaneously also optimising the PF-out, and it does not include a method for optimally determining or setting the parameters. In fact, the locus of the operation points of the OCC single stage inverter is identified by a static curve in the p-v (power-voltage) plane that does not intersect the p-v curve of the photovoltaic field in points corresponding to the maximum power points related to the various sunlight levels, except for a particular and unpredictable sunlight value.